vinces99 writes: It is well known to scientists that the three common phases of water – ice, liquid and vapor – can exist stably together only at a particular temperature and pressure, called the triple point. Also well known is that the solid form of many materials can have numerous phases, but it is difficult to pinpoint the temperature and pressure for the points at which three solid phases can coexist stably. Physicists now have made the first-ever accurate determination of a solid-state triple point in a substance called vanadium dioxide, which is known for switching rapidly – in as little as one 10-trillionth of a second – from an electrical insulator to a conductor, and thus could be useful in various technologies. "These solid-state triple points are fiendishly difficult to study, essentially because the different shapes of the solid phases makes it hard for them to match up happily at their interfaces," said David Cobden, a University of Washington physics professor who is lead author of a paper about the research published in Nature. "There are, in theory, many triple points hidden inside a solid, but they are very rarely probed."Link to Original Source

My first thought on RTFS was... "it's called a triple point because there are three phases co-existing (e.g. ice, water and steam), so what's the third phase?"

FTFA:

In 1959, researchers at Bell Laboratories discovered vanadium dioxideâ(TM)s ability to rearrange electrons and shift from an insulator to a conductor, called a metal-insulator transition. Twenty years later it was discovered that there are two slightly different insulating phases.